The XMM-Newton view of IRAS 09104+4109: evidence for a changing-look Type 2 quasar?

Aims. We report on a 14 ks XMM-Newton observation of the hyperluminous infrared galaxy IRAS 09104+ 4109, which harbors a type 2 quasar in its nucleus. Our analysis was aimed at studying the properties of the absorbing matter and the Fe K complex at 6-7 keV in this source. Methods. We analyzed the spectroscopic data from the PN and the MOS cameras in the 0.4-10 keV band. We also used an archival BeppoSAX 1-50 keV observation of IRAS 09104+ 4109 to investigate possible variations of the quasar emission. Results. The X-ray emission in the EPIC band is dominated by the intra-cluster medium thermal emission. We found that the quasar contributes similar to 35% of the total flux in the 2-10 keV band. Both a transmission-(through a Compton-thin absorber with a Compton optical depth of tau(C) similar to 0.3, i. e. N-H similar to 5 x 10(23) cm(-2)) and a reflection-dominated (tau(C) > 1) model provide an excellent fit to the quasar continuum emission. However, the value measured for the EW of Fe K alpha emission line is only marginally consistent with the presence of a Compton-thick absorber in a reflection-dominated scenario, which had been suggested by a previous, marginal (i. e. 2.5 sigma) detection with the hard X-ray (15-50 keV), non-imaging BeppoSAX/PDS instrument. Moreover, the value of luminosity in the 2-10 keV band measured by the transmission-dominated model is fully consistent with that expected on the basis of the bolometric luminosity of IRAS 09104+ 4109. From the analysis of the XMM-Newton data we therefore suggest the possibility that the absorber along the line of sight to the nucleus of IRAS 09104+ 4109 is Compton-thin. Alternatively, the absorber column density could have changed from Compton-thick to-thin in the five years elapsed between the observations. If this is the case, then IRAS 09104+ 4109 is the first changing-look quasar ever detected.